The present invention relates to paper and board machines. More specifically, the present invention relates to a method and an arrangement for controlling evaporation and moisture in a multinip calender when a continuous fibrous web is calendered in calendering nips placed one after the other before the fibrous web is wound on a reel-up/winder.
Calendering is a method by means of which the properties, such as smoothness, of a web-like material, such as a paper or board web, are sought to be generally improved. In calendering the web is passed into a nip which is formed between rolls pressed against each other and in which the web is deformed by the action of temperature, moisture and nip load, in which connection the physical properties of the web can be affected by controlling the above-mentioned parameters and the time of action, and the obtained smoothness is a function of the work done to the web.
In the papermaking art, grades of ever higher quality are required today. As the running speeds required of paper machines are continuously increasing, the direction in calendering technology is more and more towards on-line solutions, which include soft calendering and multinip on-line calendering. When the aim is to make higher quality printing paper grades having a PPS surface smoothness <2 μm, such as, for example, SC-A and LWC-roto grades and glossy coated paper grades, a substantial problem is that these kinds of grades can be produced in practice only by using, after drying a fibrous web, intermediate winding and off-line supercalenders, several of said supercalenders, usually three, being used side by side to meet production capacity.                Supercalendering is calendering in a calender unit in which nips are formed between a smooth-surface press roll, such as a metal roll, and a roll covered with a resilient cover, such as a polymer roll. The resilient-surface roll adapts itself to the contours of the surface of paper and presses the opposite side of paper evenly against the smooth-surface press roll. Today, the supercalender typically comprises 10-12 nips and for the purpose of treating the sides of the web, the supercalender comprises a so-called reversing nip in which there are two resilient-surface rolls against each other. Supercalendering is an off-line calendering method, and at the moment it provides the best paper qualities having a PPS surface smoothness <1.5 μm, such as, for example, WFC, LWC-roto and. SC-A.        Multinip on-line calendering is calendering in a calender unit in which nips are formed between a smooth-surface press roll, such as a metal roll, and a roll covered with a resilient cover, such as a polymer roll, which rolls are placed alternately one after the other. The resilient-surface roll conforms to the contours of the surface of paper and presses the opposite side of paper evenly against the smooth-surface press roll. A multinip on-line calender unit typically comprises 8 rolls and 7 nips. Linear load increases in the multinip on-line calender, in the same manner as in the supercalender, from the top nip to the bottom nip because of the force of gravity. Multinip on-line calendering is a calendering method by means of which it is possible to produce grades having a PPS surface smoothness >1.0 μm, such as, for example, film coated LWC and SC-C as well as lower-quality offset LWC and SC-B.        Soft calendering is calendering in a calender unit in which nips are formed between a smooth-surface press roll, such as a metal roll, and a roll covered with a resilient cover, such as a polymer roll. In a soft calender, the nips are formed between separate roll pairs. In order to treat both sides of the web in the soft calender, the order of the roll pairs forming the successive nips is inverted with respect to the web so that the resilient-surface roll may be caused to work on both surfaces of the web. Soft calendering is an on-line calendering method by means of which it is possible to produce grades having a PPS surface smoothness >1.5 μm, such as, for example, MFC and lower-quality film coated LWC as well as SC-C.        
Linear load increases in multinip calenders from the top nip to the bottom nip because of the force of gravity. In order to eliminate this downwardly increasing linear load, to control the deflection line of the roll, and also to quickly open the set of rolls, today's multiroll calenders employ roll relieving which is accomplished by means of a cylinder and lever arm mechanism and which compensates for the force of gravity. One such relieving system for rolls is provided in OptiLoad™ calenders.
Smoothness/work done on OptiLoad™ calenders roughly complies with the pattern shown in the graph of FIG. 2.
By means of the initial moisture content of the web before the calender and by means of the calendering temperature and steam treatments of the web the smoothness/impulse curve can be displaced, in particular in the temperature range of 100 EC-150 EC, typically by 0.2 μm in the smoothness scale in its direction.
Today, calendering problems are mainly caused by the following matters.    a. Initial moisture content, the number of steam treatments and calendering temperature are mainly determined on the basis of the final moisture content after calendering such that            i. when the final moisture content is too low, the web absorbs moisture, which results in deterioration of the achieved gloss in the form of after-roughening, and        ii. when the final moisture content is too high, the drying of the web effectively destroys the obtained quality values.            b. On the other hand, determination of the initial moisture content in calendering is affected by the desired optical properties and the level of blackening. When the final moisture content becomes too high, the opacity, or translucence, of the web deteriorates, which appears in finished paper product as an increase in print-through values, and the level of blackening rises, which diminishes the selling value of paper in the form of reduced brightness and poor visual impression.
Because of these matters, the real control variables of a modern calender are relatively limited and the operating window of a single calender has become relatively narrow with increasing drying capacity of the calender. Today, quality can be successfully improved in practice only by increasing the number of nips of the calender. In connection with this, the controllability problem is aggravated by the fact that with increasing number of nips, difficulties also increase in setting the initial moisture content and initial temperature of the web such that curl of the web is avoided and that the web is still sufficiently moist in the lowermost nips of the calender and thus mouldable, which is of high significance for achieving smoothness in particular and also density.
In known multinip calenders, the web is usually passed from one nip to another by means of take-out or turning rolls, which are each situated at the take-out of the nip. It is also known that in connection with the take-out of the nip there are provided different steam boxes, spray devices and equivalent, by which attempts are made to control the change of the moisture content of the web.
Today, the final and initial moisture contents are largely dependent on the properties of fibre material and on the functional properties required of the end product, and since the best result is achieved by simultaneously controlling the calendering and final moisture content, which should be close to the equilibrium moisture content in a situation of final use in order to avoid large roughening and dimensional change effects, the primary object of the invention is not only to reduce the above-noted drawbacks and problems associated with calendering but also to generally improve control of evaporation and moisture in the calender in order to increase the quality potential at a given impulse level. Evaporation and drying of the web occurring in different running situations are strongly dependent on running speed, linear load and temperature, wherefore moisturizing and, thus, final quality and final moisture content are very difficult to control in different situations when there is a change in the calender. For this reason, an object of the invention is also to improve controllability in order that the moisture content of the web might be controlled in different situations of operation of the calender, for example, when there are changes in speed, roll temperatures and linear load.